Possible featureless spin liquid states in high magnetic field — magnetization plateaus revisited

Contrary to a text-book picture of magnetization processes,
many examples are known both in theory and in experiment
to exhibit step-like structures (magnetization plateaus),
which are known to be "quantized" at rational values of magnetization.
The appearance of these plateaus is a direct consequence of
the formation of spin-gapped states and simple arguments tell us
that normally these spin-gap states are either trivial (i.e.
band-insulator-like)
or accompanied by magnetic superstructures.

We re-examine this issue from an effective-field-theory viewpoint.
By using semiclassical approach, we derive an effective hamiltonian
for generic (antiferromagnetic) spin systems in high magnetic
fields. The resulting model contains all-important Berry-phase term,
whose topological interference determines whether plateaus are allowed
at a given value of magnetization. On the other hand, this
interference effects may cause multiple-vortex condensation
and thereby leads to genuine featureless spin-liquid plateau states
with topological orders.